Electric displacement control for an open circuit variable displacement pump

ABSTRACT

An electric displacement control system has a hydraulic variable displacement pump that operates in an open hydraulic circuit. A servo piston is disposed within a servo bore that is connected to the hydraulic variable displacement pump. Located in the servo bore is a control spool valve having an orifice that vents fluid pressure from the servo bore to a pump case. The flow rate of the system depends upon a fluid force between a feedback spring on a first side of the control spool valve and a solenoid actuator force on a second side of the control spool valve.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the priority of U.S. Provisional Application No. 62/884,380 filed on Aug. 8, 2019, the contents of this application is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is directed to a control system for a hydraulic variable displacement pump, and more particularly a control system for an open circuit variable displacement pump which is adjustable by means of a servo piston that shifts inside of a servo cylinder.

Control systems for an open circuit variable displacement pump are known in the art. In one example, as disclosed in U.S. Publication No. 2015/0050165 by Zavadinka, a control device is used to set various power levels for the variable displacement pump that are reliably maintained by automatically regulating the control device without requiring any external control intervention. Electric displacement controls (EDC) for a pump regulate the speed of a machine function such as speed of a conveyor belt or drilling speed on a drill rig. EDCs typically replace PVG-like valves allowing cost reductions as well as lowering the hydraulic losses across the valve for better system efficiency. While useful, problems still exist with respect to cost and space while still meeting application requirements.

A solution to these problems involves an EDC concept that includes use of a two-way, two or three position, spool type valve located in the pump servo bore and having an orifice to vent fluid from the servo bore to the pump case. The spool type valve meters fluid from system pressure to the servo bore and the flow rate across the valve depends on a force balanced between a feedback spring force on one side of the spool and a solenoid actuator force on another side of the spool.

An objective of the present invention is to provide an electric displacement system that reduces the cost of manufacturing and still meets application requirements.

Another objective of the present invention is to provide an electric displacement system that reduces the space needed and still meets application requirements.

These and other objectives will be apparent to those having skill in the art based upon the following written description, drawings, and claims.

SUMMARY OF THE INVENTION

An electric displacement control system has a hydraulic variable displacement pump that operates in an open hydraulic circuit. A servo piston is disposed within a servo bore that is connected to the hydraulic variable displacement pump. Located in the servo bore is a control spool valve having an orifice that vents fluid pressure from the servo bore to a pump case. The flow rate of the system depends upon a fluid force between a feedback spring on a first side of the control spool valve and a solenoid actuator force on a second side of the control spool valve.

The control spool valve is located in a housing and has a plug with a system pressure access to a porting hole, a moveable control spool, a feedback compression spring at a first side and a solenoid actuator at a second side. Preferably the control spool valve is a two-way, two-position spool type valve or a two-way, three-position spool type valve. The control spool valve has a groove where fluid in the groove is connected with a fluid in the servo bore and fluid volume at a solenoid actuator. An axis of the of the control spool valve is aligned with an axis of the servo bore. Positioned between the servo piston and a first side of the control spool valve is a feedback spring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an electric displacement control system; and

FIG. 2 is a schematic view of an electric displacement control system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the figures, an electric displacement control system 10 includes a variable displacement pump 12. The variable displacement pump 12 has an inlet 14 and an outlet 16. The outlet 16 is connected to a control device 18 via a pressure line 20.

Control device 18 has a control piston 22 mounted within a housing 24 and is adapted to shift or move longitudinally. A first end 26 of the control piston 22 has an actuator 27 that is exposed to high pressure fluid that exerts hydraulic force toward a second end 28 having an adjustable spring 30.

The housing 24 has a first port 32, a second port 34, and a third port 36. The first port 32 is positioned to selectively align with pressure line 20. The second port 34 is positioned to selectively align with drain line 37 that extends between the control device 18 and a tank 38. The third port 36 is positioned to selectively align with pressure line 40. The control device 18 has a first position that permits flow through the control device 18 between drain line 36 and pressure line 40. The control device 18 has a second position where fluid flows from pressure line 20 to pressure line 40.

The pressure line 40 extends from the control device 18 and a spool type valve 42. The spool type valve 42 is of any type and preferably is a two-way two position spool valve 42A for a zero flow and minimum fail-safe control design, or a two-way three position spool valve 42B for a full flow and maximum pressure fail-safe control design.

The spool valve 42 is located in a pump servo bore 44 and the axis of the spool valve 42 is aligned with the axis of the servo bore 44. The spool valve 42 includes a plug 46 with a system pressure access to a porting hole 48, a moveable control spool 50, a feedback compression spring 52 on one end, and a solenoid actuator 54 on the opposite end. A groove 56 on the control spool 50 selectively permits flow from the outlet 16, through the control spool 50 to pressure line 40. The two-way three position spool valve 42B has a pair of plugs 46A and 46B on each side of the groove 56.

The servo bore 44 has an orifice 58 to maintain a pressure differential between servo pressure and case pressure. Fluid pressure from the spool valve 42 acts upon a servo piston 60 moving the piston 60 against the force of a return spring 62. Movement of the servo piston 60 adjusts the variable displacement pump 12 as required via an activation link 64. This adjustment results in a change in the deflection angle of a swashplate.

In operation, using the two-way two position spool valve 42A, with no or minimum solenoid force, fluid pressure flows from outlet 16, through control spool 50 to pressure line 40. This results in sending the pump 12 to minimum displacement.

As solenoid force increases, the spool valve 42A moves toward and against feedback spring 52 reducing flow to the servo. As a result, the pump bias system returns pump 12 displacement to maximum while compressing the feedback spring 52 until the spring force and the actuator force are balanced.

When the actuator force is decreased, moveable control 50 is pushed by feedback spring 56 against the solenoid actuator 54 and system pressure is communicated to the servo piston 60. The servo piston 60 moves out of the servo bore 44 and force from the feedback spring 56 is lowered until spring force and solenoid actuator force are balanced again. With maximum solenoid force, system pressure is blocked at porting hole 48 and fluid in servo bore is all vented to case as the pump bias system returns the pump 12 to maximum displacement.

The two-way three-position spool valve 42B operates in a similar manner to move the control member 50 between maximum and minimum displacement, but also provides an emergency or fail safe when electrical power to the solenoid actuator 54 is lost. More specifically, when solenoid force drops below a preset minimum force from the feedback spring 52 moves the control member 50 all the way against the solenoid actuator which blocks system pressure. Pump displacement would be returned to maximum as all fluid would be vented to the pump case through the orifice. 

What is claimed is:
 1. An electric displacement control system, comprising: a hydraulic variable displacement pump operated in an open hydraulic circuit; a servo piston disposed within a servo bore connected to the hydraulic variable displacement pump; a control spool valve located in the servo bore and having an orifice that vents fluid pressure from the servo bore to a pump case.
 2. The control system of claim 1 wherein flow rate depends upon a fluid force between a feedback spring on a first side of the control spool valve and a solenoid actuator force on a second side of the control spool valve.
 3. The control system of claim 1 wherein the control spool valve is located in a housing and has a plug with a system pressure access to a porting hole, a moveable control spool, a feedback compression spring at a first side and a solenoid actuator at a second side.
 4. The control system of claim 1 wherein the control spool valve is a two-way two-position spool type valve.
 5. The control system of claim 1 wherein the control spool valve is a two-way three-position spool type valve.
 6. The control system of claim 1 wherein the control spool valve has a groove where fluid in the groove is connected with fluid in the servo bore and fluid volume at a solenoid actuator.
 7. The system of claim 1 wherein an axis of the control spool valve is aligned with an axis of the servo bore.
 8. The system of claim 1 wherein a feedback spring is positioned between the servo piston and a first side of the control spool valve.
 9. A method of controlling the displacement of a variable displacement pump, comprising the steps of: providing a hydraulic control system that includes a spool type valve having an axis aligned with an axis of a servo bore, wherein the spool type valve has a solenoid actuator on one end and a feedback spring on an opposite end; communicating system pressure through the spool type valve with servo pressure when there is no solenoid force acting upon the spool type valve sending a variable displacement pump to minimum displacement.
 10. The method of claim 9 further comprising the step of increasing solenoid force to move the spool type valve against feedback spring force reducing flow to a servo control to send pump displacement to maximum until the feedback spring force and solenoid force are balanced.
 11. The method of claim 10 further comprising the step of decreasing solenoid force so that the spool type valve is pushed by the feedback spring force against the solenoid force.
 12. The method of claim 9 wherein the spool type valve is a two-direction three position spool valve that blocks system pressure when solenoid force drops below a preset minimum. 